Method and apparatus for developing electrostatic image



Nov. 17, 1959 E. F. MAYER ETAL 2,913,353

METHOD AND APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGE Filed Feb. 8. 1955 IN VEN RD F. MAY RT T. JOY

EDWA BY ROBE LAWRENCE l. FIELD ATTORNEY United States Patent METHOD AND APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGE Edward F. Mayer, Cleveland, and Robert T. Joy, Willowick, Ohio, assignors, by mesne assignments, to General Dynamics Corporation, San Diego, Calif., a corporation of Delaware Application February 8, 1955, Serial No. 486,806

3 Claims. (Cl. 117-37) This invention relates to electrostatic printing and electrooptical photography. More particularly, it relates to a method and apparatus for developing a visual image from a latent electrostatic image in such a fashion and by such means that either a positive or negative of the latent image may be produced.

The art of electrophotography is of comparatively recent origin. One interesting aspect of this art is embodied in the US. Patents issued to Chester F. Carlson, namely 2,221,776; 2,277,013; 2,297,691 and in others. In these patents there are described various methods and means for producing a latent electrostatic image in the form of a charge pattern formed on or trapped in a photoconductive layer. in the procedure outlined in these patents, the latent electrostatic image is converted to a visual image by contacting the latent electrostatic image with a suspension of dry powdered material or With a fine mist of ink. Another recent development in this field is described in the copending applications of Edward F. Mayer, particularly Serial No. 380,285, filed September 15, 1953, and Serial No. 406,192, filed January 26, 1954. As disclosed in these applications, a latent electrostatic image produced by methods which are distinctly different from those disclosed by Carlson in the aforesaid patents is developed into a visual image by contact with either a suspension in air of a powder or mist as in Carlson or by contact with a dispersion of finely divided solid particles in a body of liquid.

One serious disadvantage inherent in the above-identified prior art is an inability to convert readily from the production of visual positives into visual negatives except by changing the color of the particles employed, e.g. by printing with a white powder on black paper, or by otherwise changing the final printing means.

We have now developed a practical and relatively simple system for producing either positive or negative visual images of the image from which the latent electrostatic image was obtained. Our invention, which embraces both the method and the apparatus for obtaining a visual image from the latent electrostatic image involves the use of a dispersion of solid particles in a body of liquid, similar to those described in the aforesaid copending applications of E. F. Mayer. Our method is equally applicable to other liquid developers and is preferably employed in conjunction with a dispersion of finely divided solid particles in an organic liquid to which an aerogel has been added for the purpose of thickening the developer and to minimize or entirely prevent any tendency of the solid particles to settle or collect as clumps, clusters or other forms of aggregations.

In accordance with our invention, instead of merely bringing the photoconductive layer containing the latent electrostatic image into contact with a dispersion of a solid particulate material, as in the aforesaid copending patent applications, the element containing the latent image, the tank containing the dispersion and the applicator by means of which the dispersion is applied to the- 2,913,353 Patented Nov. 17, 1959 latent image are all suitably electrically connected so as to form a system in which the polarity or relative potential of the member bearing the latent electrostatic image may be altered relative to the applicator. By constructing the apparatus from materials having the appropriate electrical properties, and by the use of a simple switch means, either a positive or negative visual image may be obtained.

The electrostatic printing apparatus that we have devised comprises principally a means for developing a visual image from a latent electrostatic image. The developer means for developing a visual image from the electrostatic on the photoconductor comprises an applicator, preferably in the form of a roller which is partially immersed in a dispersion of particulate material. Once the latent image has been converted to a visual image, the visual image may be permanently recorded on a sheet of image-recording material by any conventional technique known in the art.

Our process for electrostatic printing comprises establishing an electrostatic charge pattern on a surface of photoconductive material, bringing the surface of a semiconducting material into intimate contact with the electrostatically charged surface, applying finely divided particles of an opaque material to the charged surface of the semi-conducting material and thereby developing a visual image thereon corresponding to the electrostatic charge pattern on the surface of the photoconductive material, and transferring the visual image thus developed to the surface of a sheet of image recording material.

As hereinabove noted, the latent electrostatic image may be produced by any suitable process. Thus we may produce the latent image by the methods disclosed in the above-mentioned Carlson patents or according to the methods set forth in the above noted Mayer applications. Whatever the method employed, there is produced on the surface of an electrophotographic or electrooptical plate, a charge pattern corresponding in configuration and intensity to the visual light-and-shadow image ordinarily produced. By the use of techniques herein disclosed, we have found it possible to develop the electrostatic charge pattern into either a positive or negative visual image that may be made into a permanent record of the image originally projected onto the photoconductive surface.

The electrostatic charge pattern on the surface of the electrophotographic plate is developed into a visual image corresponding thereto by applying to the surface of the plate finely divided particles of a dispersed powder such as charcoal, coal, colored glasses, resins or the like. These finely divided particles may be applied to the photographic plate in the form of a dispersion in a suitable dielectric liquid. To apply the particles to the photographic plate, we have found that a roller of semiconductive material, partially immersed in the dispersion is an excellent applicator. The roller, which may be either power driven or manually driven, is rotated a sufficient number of times to cause a uniform film of the dispersion to form thereon. Thereafter, the electrophotographic plate bearing a latent electrostatic image is brought into intimate contact with the roller. When this is done the charge on the electrophotographic plate, that is the latent electrostatic image, creates a field between the surface of the plate and the surface of the roller. This causes the particles in the developer dispersion to precipitate out onto the electrophotographic plate in a pattern correspending in density and configuration to the charge pat tern constituting the latent image. The image-developing liquid dispersion that We prefer to use comprises a dispersion of finely divided particles in a suitable liquid, for instance a liquid hydrocarbon or chlorinated hydrocarbon boiling at temperatures between about C. and

C. 'As above noted, the liquidmay be thickened by the use of an aerogel to inhibit any tendency of the particles to agglomerate or to settle. The particles may be between 0.01 micron and 40 microns in diameter. For most purposes particles 1 to v1O microns are suitable, although when a very high degree of resolution is desired, the particle size may be within the range of 0.01 to'0.5 micron in diameter.

The visual image formed on the surface of the electrophotographic plate may be transferred to a sheet of imagerecording material such as paper coated with an appropriate adhesive, to form thereon a light-and-shadow reproduction of the electrostatic image originally existing on the electrophotographic plate.

A preferred embodiment of our invention is shown in the single figure which is a schematic side view of the developer apparatus.

An electrophotographic plate 14) carrying a latent electrostatic image to be converted to a visual image is shown as composed of a conductive backing material, such as a thin metal film 12 coated with a thin layer of a high resistance photoconductor 14. An electrical connection 16 is provided for connecting the metal film 12' to ground.

The developer means includes a tank 20 containing a dispersion 24 o-f'finely divided particles in any suitable liquid. Tank 29 is connected to ground by a lead 22. An applicator roller 26 supported by means not shown is partially immersed in the liquid. Roller 26 consists of a metal core 28 covered by a semiconducting composition layer 30. Layer 34) may be of any suitable material. We have found that materials having a resistance between 10 and 10 ohms are particularly useful for layer 30. Roller 26 is completely insulated from ground and is provided with an electrical connection 32 and a switch 34 by means of which it may be connected and disconnected from ground. A second roller 40, is mounted so as to rotate in contact with roller 26. Suitable means such as spring loading may be provided for urging roller 40 toward roller 26 so that any electrophotographic plates passed between the rollers will be intimately contacted by roller 26, which may be either cranked by hand or driven by suitable power means.

The operation of the apparatus is as follows. Tank 20 is filled with a dispersion of finely divided solid particles in a suitable liquid. Roller 26 is caused to rotate whereupon a thin film of the dispersion is carried up the surface of the roller. The roller is turned through several rotations in order that it may become uniformly coated with the developerv dispersion. An electrophotographic plate bearing a latent electrostatic image is then inserted into the bite of the rolls and is drawn between them and passed to the opposite side. While this is done the charge on the electrophotographic plate creates a field between the surface of the plate and the surfaceof the roller 26. This causes the particulate material dispersed in the carrying liquid to precipitate out on to the surface of the electrophotographic plate. The number of particles which precipitate in any area is determined by the charge upon that area. Hence a powder pattern is built up on the surface of the photographic plate which is inversely proportional to the original light image. The powder pattern may be removed by any of several techniques well known in this art whereby it may be converted into a permanent image.

In accordance with our invention, it is now possible to obtain either a positive of the original image or a negative of the image at will. To obtain a positive of the image, the above-described apparatus is operated with switch 34 closed, in which instance roller 26 and the plate 12 are both connected to ground. If switch 34 is opened, the powder deposition will occur in the reverse manner.- The deposition of powder from the dispersion remains inversely proportional to the charge on the surface of the photographic plate, but with the switch 34 open, the

4 maximum deposit will occur on the completely discharged areas, and the minimum deposit will'occur on the originally charged areas. There will therefore be produced with the switch 34 in the open position, a negative image of the light pattern originally impressed upon electrophotographic plate 10.

In effect then roll 26 either may be grounded or may float at a potential equal to the averagecharge on the photoconductor 14. With switch 34 closed and with both metal core 28 and conductive film 12 connected to ground, the maximum powderdeposit occurs on the areas of photoconductor 14 having the maximum charge because the electrostatic field is strongest in these areas. With switch 34 open, some of the charge on the high resistance photoconductor 14 will be imparted to roller 26 causing it to fioat at a potential equal to the average charge on the photoconductor. In this instance the powder will deposit on the areas of the photoconductor which were discharged when the latent electrostatic image was formed since in these areas the field will now be strongest. The minimum deposit of powder will occur in the areas of maximum charge because in this arrangement the field is now weakest. Accordingly, it will be seen that the desired reversal of the visual image is readily obtained by either grounding roller 26 or by opening switch 34.

We claim:

1. An apparatus for developing a visual image from a latent electrostatic image existing on: an electrophotographic plate consisting of a photoconductive surface bearing the latent electrostatic image and supported on an electrically conductive backing member which comprises: means electrically connecting said backing member to ground; a tank adapted to contain a dispersion of solid opaque particles dispersed in an electrically insulating liquid; 'means electrically connecting said tank to ground; a rotatable applicator formed of an electrically semiconductive outer layer covering an electrically conductive supporting core; switch means for selectively making and breaking an electrical circuit between said electrically conductive core and ground; means for providing a thin film of said dispersion to the semiconductive surface layer'of said applicator; and means for bringing said film-bearing surface into intimate physical contact with the photoconductive member bearing the latent electrostatic image in order to render the latent image visible.

2. A method of developing a latent electrostatic image on the surface of a photoconductive material supported on an electrically conductive backing member maintained at ground potential which comprises: coating the surface of a cylindrical applicator formed of an electrically conductive core covered by electrically semiconductive material with a thin uniform film of a dispersion of finely divided solid, opaque, electrostatically attractable particles dispersed in an electrically insulating liquid; and bringing the coating on said coated applicator into physical contact with the surface bearing the latent electrostatic image thereby simultaneously establishing an electrical field between said applicator and said latent image bearing member by virtue of the electrostatic charge on saidlatent image bearing member and depositing finely divided solid opaque particles from said thin film onto the surface of the photoconductive material in proportion to the strength of the electrical charges in the latent image, said polarity of said field being determined by electrically connecting the electrically conductive portions of said applicator to ground potential during the interval in which said latent image bearing member and said film bearing member are in physical contact.

3. A method of developing a latent electrostatic image on the surface of a photoconductive material supported on an electrically conductive backing member maintained at ground potential which comprises: coating the surface of a cylindricalapplicator formed of an electrically conductive co're'covered by electrically semiconductive material with a thinuniform film of a dispersion of finely divided solid, opaque, electrostatically attractable particles dispersed in an electrically insulating liquid; and bringing the coating on said coated applicator into physical contact with the surface bearing the latent electrostatic image thereby simultaneously establishing an electrical field between said applicator and said latent image bearing member by virtue of the electrostatic charge on said latent image bearing member and depositing finely divided solid opaque particles from said thin film onto the surface of the photoconductive material in proportion to the strength of the electrical charges in said latent image, the polarity of said field being determined by electrically insulating the electrically conductive portions of said applicator from ground potential during the interval in which said latent image bearing member and said film bearing memher are in physical contact.

References Cited in the file of this patent UNITED STATES PATENTS Flitcroft Sept. 3, 1901 Gillespi Dec. 8, 1914 Carlson Nov. 19, 1940 Houper Aug. 29, 1950 Hooper July 3, 1951 Steinhilper July 31, 1956 Walkup Mar. 5, 1957 Greig Oct. 29, 1957 FOREIGN PATENTS Great Britain Oct. 23, 1922 Great Britain Oct. 28, 1953 France Dec. 30, 1953 

1. AN APPARATUS FOR DEVELOPING A VISUAL IMAGE FROM A LATENT ELECTROSTATIC IMAGE EXISTING ON: AN ELECTROPHOTOGRAPHIC PLATE CONSISTING OF A PHOTOCONDUCTIVE SURFACE BEARING THE LATENT ELECTROSTATIC IMAGE AND SUPPORTED ON AN ELECTRICALLY CONDUCTIVE BACKING MEMBER WHICH COMPRISES: MEANS ELECTRICALLY CONNECTING SAID BACKING MEMBER TO GROUND; A TANK ADAPTED TO CONTAIN A DISPERSION OF SOLID OPAQUE PARTICLES DISPERED IN AN ELECTRICALLY INSULATING LIQUID; MEANS ELECTRICALLY CONNECTING SAID TANK TO GROUND; A ROTATABLE APPLICATOR FORMED OF AN ELECTRICALLY SEMICONDUCTIVE OUTER LAYER CONVERING AN ELECTRICALLY CONDUCTIVE SUPPORTING CORE; SWITCH MEANS FOR SELECTIVELY MAKING AND BREAKING AN ELECTRICAL CIRCUIT BETWEEN SAID ELECTRICALLY CONDUCTIVE CORE AND GROUND; MEANS FOR PROVIDING A THIN FILM OF SAID DISPERSION TO THE SEMICONDUCTIVE SURFACE LAYER OF SAID APPLICATOR; AND MEANS FOR BRINGING SAID FILM-BEARING SURFACE INTO INTIMATE PHYSICAL CONTACT WITH THE PHOTOCONDUCTIVE MEMBER BEARING THE LATENT ELECTROSTATIC IMAGE IN ORDER TO RENDER THE LATENT IMAGE VISIBLE. 